Add terraform-ipam CI Vault access (#110)

Grants the `terraform-ipam` Woodpecker pipeline its Vault footprint, mirroring the `terraform-artifactapi` set plus a KV read for provider tokens.

- k8s auth role `woodpecker_terraform_ipam` (SA `terraform-ipam`, ns `woodpecker`)
- consul secret backend role `terraform-ipam` + ACL policy writing `infra/terraform/ipam/*`
- consul creds read policy
- kv read policy for `kv/service/terraform/ipam` (`netbox_token` + `kea_token`)

The KV secret itself still needs seeding before the pipeline can auth to NetBox/Kea.

https://claude.ai/code/session_01JUoARVdmhxKQHyyyp1pxeT
Reviewed-on: #110
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

resources/secret_backend/consul_root/au/syd1/terraform-ipam.hcl

@@ -0,0 +1,7 @@
+key_prefix "infra/terraform/ipam/" {
+  policy = "write"
+}
+
+session_prefix "" {
+  policy = "write"
+}